An overview of PIXE
and its application


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Islamic University Islamabad


This article will
discuss the brief introduction to the PIXE technique and its applications in
different fields of life. PIXE method is frequently referred to as
Proton-induced X-ray emission, due to the fact that, in this technique, protons
are commonly chosen to induce X-ray emission. Around the world PIXE is
regularly applied for numerous interdisciplinary problems: Environmental
studies, investigation of materials, in biomedicine, archeometry and geology etc.
The vast applications of Particle Induced X-ray Emission (PIXE) has led to a significant
attesntion to the research.




Proton Induced X-ray Emission (PIXE) was first introduced as a
universal analytical technique by Johansson et al. 1 at the Lund institute of
technology in the early 1970s .Many analytical techniques were used to trace
element analysis, in comparison with PIXE it was observed that their sample
preparation was generally complicated and takes a long time. The popularity and
range of the PIXE applications has continuously increased over the years.
Currently, throughout the world several laboratories are using PIXE for
elemental analysis in diverse fields as environmental studies, biology
medicine, materials sciences, earth sciences etc 2-4. Conferences on PIXE and
its use in applications are been held after every third year since 1977.




The ability of
non-destructive methods to perform multielemental determinations has led to an
extensive applications in different industrial and research laboratories. Proton
Induced X-ray Emission (PIXE) is one of the most precise and sensitive atomic and
nuclear analytical technique 5-7. Particle Induced X-ray Emission (PIXE) is
PIXE is a technique used for the elemental analysis. The sample preparation of
PIXE are generally simple, making it a very less time consuming technique. The
Proton-induced X-ray emission is the most popularly used technique in the
category of ion beam analysis 8. The accuracy and sensitivity of
Proton-induced X-ray emission is much better for analysis of both matrix and
trace elements in thick samples as compared with other ion beam techniques 1.
IN comparison with electron beam induced x-ray spectroscopy PIXE gives a much
higher signal to background. PIXE is 100 times more sensitive compared to
energy dispersive X-ray spectroscopy. Due to its non-destructive and
multi-element analytical capability Proton-induced X-ray emission has found wide
applicability in various research fields. The popularity and the great
potential to meet the current demand of trace elements analysis with PIXE is
increasing day by day 9.

Principle of PIXE


Proton Induced X-ray
Emission (PIXE) mainly consists of two main parts. In the first part
identification of the atomic species in the target from the energies of the
characteristics peaks in the X-ray emission spectrum is done, while in the
second part the amount of the element present in the target from the intensity
of its characteristic X-ray emission spectrum is determined. The knowledge of
the ionization cross-sections, fluorescence yields and absorption coefficients
is normally required for this process. By exciting the target atoms induced by
an energetic ion beam of protons or alpha particles X-rays can be produced as
shown in figure 1 10.

In the PIXE technique the
element is identified by the energy of the characteristic X-rays and elemental
concentration for specific element gives us information about the number of
emitted X-rays with characteristic energy peak 11.

In PIXE the
samples are bombarded with high-energy protons accelerated in an accelerator
.These high energy protons strike the target atoms and electrons from the inner
shell are ejected. As a result, in the innermost shell a vacancy is created.
When protons interact with matter it results in the production of an X-Ray
spectrum by the de-excitation of the atoms in the sample. The de-excitations of
the atoms can also occur due to the emission of an electron, called Auger
electron. The electronic transition that take place may be accompanied by the
emission of electromagnetic radiation in the form of x-ray spectrum of the
excited atom. This emission of electromagnetic radiation in the form of X-rays
characteristic is consist of K,L,M lines which are produced by the electronic
transitions to the K,L,M.. Shells of the target atom. By doing so elemental
composition analysis can be achieved by determining the X-ray emissions
originating from the excited sample. The nondestructive  nature of this technique and high sensitivity
factor has made this technique as one of the most suited to study the trace
element content of air particulate, analysis of soil, sediments and plants.


                                                       Figure 1. The
basic principle of PIXE.

Ref 10                                                                                                                          

Applications of PIXE


The analytical
applications of PIXE are countless. Due to its non-destructive nature of the
technique, PIXE is currently applied to a wide range of material analysis in
different research laboratories all around the globe. The following discussion
will outline the different group of applications and different aspects of PIXE
that are important for its application in various fields.



pollution Studies were among the first applications to be successfully
performed during the early years of PIXE development. Currently investigation
of water pollution and air pollution is been done with the help of PIXE, with
more emphasis on the latter. The aerosol analysis is significantly important
for the air-quality assessment in urban and industrial areas as its impact on
the health of all living organisms is important. Using PIXE for aerosol
analysis provides an important tool for the investigation of many atmospheric phenomena’s
such as air transportation, visibility and climate. Currently widespread use of
aerosol investigations by PIXE has become apparent.   Every element is potentially important in
aerosol analysis, it can be because of their toxic nature or maybe because any
of such compounds bound in particles with the help of scattering of light can affect
the visibility, also the simultaneous detection of group elements can also be a
sign of pollution. By performing the multielemental quantitative analysis PIXE
can be an important tool to offer an advantage in terms of cost and time. Using
PIXE with other IBA techniques can also extend the range of detectable
elements. The information on the composition of different sizes of aerosols is
important to obtain. The relevance of particles having different sizes is very
different, and the distribution of the particle size is also non homogenous.
The impact on health is mainly connected to the penetration of the particles
into the breathing apparatus, smaller particles are potentially more dangerous
because they reach much deeper levels into the breathing apparatus. The
visibility problems created by aerosols, the dependence of their effect on size
is more obvious to be connected with the scattering of light. The distribution
of the size also gives us worthy information on the different sources of
pollution. In comparison with other techniques, the higher sensitivity of PIXE
can be exploited to the highest degree, with no other technique can provide
such a high time resolution and sensitive detection of many elements
simultaneously in a short amount of time 8.


Biomedical applications

In the last decade, PIXE techniques found great success in solving the
biological and biomedical problems. The research on biomedical samples has also
increased in the last few years. Though PIXE is still a new member of the
family of spectrographic methods but interest in this technique is growing day
by day. Apart from environmental studies majority of the PIXE applications are
found in biology and medicine. The small amount of the material and the time of
analysis are important requirements for biomedical investigations in order to
obtain significant cooleration between the trace elements and biomedical
phenomena, which can be easily carried out with the help of PIXE.

Another important
feature of using PIXE in medical applications is the high spatial resolution,
which helps in the possibility of mapping the trace-element distributions in
the tissues with a much higher definition. The intercellular distribution of
trace elements can be revealed by using a micro beam and sometimes there are no
other technique then micro-PIXE .A close interdisciplinary collaboration
between the physicists and biologists in problems like planning, sample
preparation data collection processing is important to obtain good results in
biomedical PIXE applications. From the toxicological point of view, trace
elements such as lead are of significant interest. In addition to that,
investigation for a possible link between the trace element concentrations and
various diseases are also been studied 8,10. In the past, the main focus of
biomedical PIXE applications was the detection of trace quantities of
transition elements and heavier atoms in tissues, recently higher elements like
potassium and calcium are also of great interest. When dealing with the small
tissues samples, often the low concentration levels of these elements are a
major issue in analysis by using other techniques .This problem was solved by
PIXE as the concentrations of these elements were well within reach of PIXE. Bio-PIXE
experiments are generally performed on such samples which are collected from
the living matter. Many investigations are performed on dermatological samples
like nails or hairs in order to avoid ethical problems connected with acquiring
of such samples from living organisms. Such samples also serves as biological
indicators of a physiological or pathological state of the individual from
which these samples are taken. The use of body fluids in the sampling
procedures also implies minimum invasion. The high absolute sensitivity of PIXE
is a good reason for selecting this technique while dealing with biopsies
(living matter).However, special care should be taken because of the very small
amount of sampled material involved in order to avoid contamination from the
surgical instruments 8.


Industrial applications


The industrial applications of any
accelerator-based technique depends on the availability of instrumentation. Non
availability of instruments clearly limits the growth of applications in an
industry. There are only few semiconductor industries that have in-house
particle accelerators which might improve this situation. IBA techniques had a
significant impact in the field of economic geology, by supporting trace
element data with the help of a nuclear microprobe on mineral samples 12.

Geogas prospecting is one of the
most important industrial application of PIXE. The concept of using Geogas in
prospecting is based on the emissions of small amounts from deep underground
deposits to the earth surface 13. In this method small particles are first
collected on thin foils and then are exposed for serval weeks, after this they
are analyzed using a millimeter ion beam. Composition of the material deposited
then can be revealed by PDCE. By a rather complex interpretation procedure
utilizing geophysical principles the nature mineral deposits in deep ground can
be judge. This technique has great potential in the evaluation of rock
conditions in order to investigate the suitability of sites for highly
radioactive waste storage.

Another industrial application
which is slightly different from the one discussed earlier is the use of
focused ion beams for characterization of the fibrous structure of newsprint
paper and to analyze the characteristics of printing 14.



and its scope in Pakistan


Around the world PIXE is regularly
applied for numerous research related problems. In the field of sciences and
technology Pakistan is one of the most fastly developing country, IBA
techniques are popularly used around the world, PIXE being the most popular
iron beam analysis technique has vast applications in many fields of life.


Atomic and Laser Physics laboratory in National Centre of Physics Pakistan,
tandem particle accelerator facilities are available. Proton-Induced X-Ray
Emission (PIXE) is one of the important techniques for the analysis of
materials which is offered in Atomic and Laser Physics laboratory in National
Centre of Physics Pakistan.
It has sensitivity up to 1 ppm and accuracy of 1%. Currently there are several
projects running in National Centre of Physics Pakistan for the purpose of Environmental science research.
These projects include the indoor/outdoor air pollution study with the use of
PIXE/PIGE techniques. GC University Lahore has also 4MeV pelletron accelerator
which have two active beam lines. One of these beam line is being effectively
used by different science departments of the University for the Proton-Induced
X-Ray Emission (PIXE) analysis of materials.


In Pakistan several research
scientist has used PIXE for different environmental, biomedical any many other
applications. Some of them are discussed in this article.


Nisar Ahmad et al 15 did research on the aerosol studies of urban
areas of Lahore by using the PIXE technology. The trace elements were analyzed
by using PIXE technology. It was observed that the cleaner residential areas
had lesser amount of concentration of trace elements of anthropogenic origin.
International Scientific Spring Conference which was held in Islamabad,
Pakistan in march 2010, Nisar Ahmad also presented the analysis of some Medical
herbs by using the Proton-Induced X-Ray Emission (PIXE) technique .Nisar Ahmad also
presented his research work on the Proton-Induced X-Ray Emission (PIXE) technique in the 3rdInternational
Conference on Frontiers of Advanced Engineering Materials which was organized
by PITMAEN, PCSIR, which was held at Lahore Pakistan.


 From the past
few decades the chromium-based tanning industry has grown significantly in
Pakistan. However due to the rules and regulations imposed by Pakistani
government are not strictly followed for the effluent discharge from the
tanneries. Tanner effluents has become a great source of water pollution. Lubna
shakir et al 16 studied the characterization of tannery effluent waste water
by Proton-Induced X-Ray Emission (PIXE) analysis to investigate their role in
water pollution. This case study was done in Kasur Pakistan , where
characterization of tannery effluent wastewater (TW), shallow ground water
(SW), and deep ground water (DW) were taken to determine that what are the
sources of pollution in the district Kasur Pakistan. Results showed that the concentrations
of calcium (Ca), chlorine (Cl), chromium (Cr), iron (Fe), potassium (K), Mg,
sulfur (S) silicon (Si) and Sr in Tw were much higher than SW and DW.


A.U.Rehman et al 17 studied the Elemental
Composition of Groundnut with the help of Proton-Induced X-Ray Emission (PIXE),
which highlighted the role of PIXE in elemental behavior in the field of agricultural research. The
results showed by PIXE were a lot better than other techniques.

Ali Shad
et al 18 studied the standardization of proton-induced x-ray emission
technique for analysis of thick samples. For the standardization, three
different samples were taken of standard reference materials (SRMs) and then
they were analyzed by using Proton-Induced X-Ray Emission (PIXE) technique.
After this the final data was compared with the already known data of these
certified SRMs. The final result showed that the data obtained by using PIXE
technique was almost similar to the already known data of certified SRMs.



PIXE is a multi-element non-destructive technique
which is known for its sensitivity, accuracy, precision, simplicity of target
preparation which allows the detection of the presence of many elements in a
sample in a very short amount of time. PIXE has become a very popular technique
for material analysis as it has vast applications in Environmental studies,
investigation of materials, in biomedicine, archeometry and geology etc. It is
one the best iron beam analysis technique.

 In a short span
of time PIXE has shown its versatility and usefulness by the diversity of
applications it has. In future PIXE should find itself working in more useful
applications with more variety of samples. The scope around this technique is
significantly increasing all around the world. 
Many developing countries like Pakistan are doing lot of Environmental
Research with the help of PIXE technique. For multi-elemental and
non-destructive techniques PIXE should take its rightful place among the
numerous other standard analytical techniques available.


















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